The present invention relates to a portable motor powered device such as a power tool, and more particularly to such a motor powered device which is capable of operating on either one anyone of a low DC voltage supplied from a codeless battery pack and a high DC voltage supplied from a corded power pack.
WO 89/07997 discloses a dual-mode system for motor powered device which utilizes a cordless battery pack and a corded power pack selectively for energizing a motor. The corded battery pack contains cells to provide a DC voltage to the motor. The corded power pack includes a power code for receiving an AC source voltage and a converter for converting the AC voltage to the DC voltage to be supplied to the motor. In order to make the corded power pack compatible with the cordless battery pack, the corded power pack further includes a step-down transformer. However, such transformer is generally bulky and heavy, and therefore adds extra dimension and weight to the corded power pack as compared to the cordless battery pack. This is not satisfactory when the powered device is required to be compact and lightweight enough to be completely portable.
In view of the above insufficiency, the present invention has been accomplished to provide a portable motor powered device which is capable of operating on a cordless battery pack as well as a corded power pack, yet enabling to reduce the size and weight of the corded power pack to be truly compatible with the corded battery pack in size and weight. The device in accordance with the present invention comprises a housing accommodating a DC motor. The cordless battery pack is detachably connected to the housing and has a battery which supplies a low DC voltage for operating the DC motor. The corded power pack is detachably connected to the housing and supplies a high DC voltage for operating the DC motor. The corded power pack includes a power cord adapted to be connected to receive an AC source voltage and a converter for converting the AC source voltage into the high DC voltage. The characterizing feature of the present invention resides in that the DC motor includes a rotor which is provided with a first winding and a second winding, the first winding being coupled to receive the low DC voltage for driving the DC motor, and the second winding being coupled to receive the high DC voltage for driving the motor. Thus, the DC motor can be driven by either one of the low DC voltage supplied from the cordless battery pack and the high DC voltage supplied from the corded power pack. Accordingly, the corded power pack can be designed to supply the high DC voltage directly converted from the AC voltage source and therefore dispense with a step-down transformer of bulky and heavy nature. Thus, the corded power pack can be made compact and light-weight as or even less than the cordless battery pack, so that the motor powered device attached with the corded power pack can be made compact and lightweight sufficiently to be truly portable.
Preferably, the first and second windings are designed to give the same torque-speed characteristic when operated at the low DC voltage and at the high DC voltage, respectively for assuring consistent work by the device.
The housing includes a first electrical path connecting the low DC voltage to the first winding and a second electrical path connecting the high DC voltage to the second winding. The first and second electrical paths are preferred to be separated from each other so that they can be designed independently from each other and therefore in accordance with specific requirements as to insulation distance and current carrying capacity. That is, the second electrical path, which carries a small current from the high DC voltage, can be designed to be have reduced conductor""s size as compared to the first electrical path carrying a large current from the low DC voltage. And, the first electrical path, which carries a large current from the low DC voltage, can be designed to have a short insulation distance as compared to the second electrical path carrying a small current from the high DC voltage.
It is preferred that the first winding is designed to operate on the low DC voltage of 2.4 to 48 volts, while the second winding is designed to operate on the high DC voltage of 100 to 300 volts.
Also, the cordless battery pack and the corded power pack may have respective casings of similar shape which are selectively attached to the housing.
The casing of the cordless battery pack includes a first plug for insertion into a socket of the housing, and the casing of the corded power pack includes a second plug for insertion into the same socket. The first plug has a low voltage contact for electrical connection with a first terminal provided in the socket, while the second plug having a high voltage connector for electrical connection with a second terminal provided in the socket.
The corded power pack may include a ground lead extending therefrom for connection to the ground. The second connector has a ground terminal for electrical connection between the motor and the ground line. Thus, the motor power device can be grounded when using the high DC voltage from the corded power pack.
Further, the corded power pack includes a circuit board on which the converter is mounted. The circuit board may include a grounding line for connection between the ground terminal and the ground lead. The ground line is connected to a noise reduction capacitor on the printed circuit board.
Preferably, one end of the power cord is supported to the casing of the corded power pack to pivot about a pivot axis so that a user is easy to operate the device without being hindered by the power cord. In this connection, the corded power pack may further include a click mechanism for retaining the one end of the power cord at least one angular position around the pivot axis. Thus, the device can be stored easily in place with the power cord being latched.
The device may include a speed sensor which is connectable to non-energized one of the first and second windings to detect an alternating voltage developed across the non-energized winding one of said first and second windings for sensing a rotation speed of the motor while the other one of the first and second windings is energized to drive the motor. Thus, the non-energized one of the first and second windings can be best utilized for controlling the rotation speed of the motor.
The housing includes a power switch for connecting the low and high DC voltages to the first and second windings, respectively. The rotor includes a core provided with the first and second windings, and a rotor shaft extending through the core. The rotor shaft carries first and second commutators respectively connected to the first and second windings. The first and second commutators are preferably disposed on opposite sides of the core to be spaced axially along the rotor shaft for contact with first and second brushes which supply currents respectively from the low and high DC voltages. The first brush is connected to the power switch through a low voltage line, while the second brush is connected to the power switch through a high voltage line. The low voltage line is preferably routed within the housing a shorter distance than the high voltage line. Thus, the low voltage lime carrying a relatively large current can reduce ohmic loss.
Further, the rotor shaft is preferred to carry a cooling fan which is disposed adjacent to the first brush for effectively cooling the first brush carrying the large current and having contact resistance with the first commutator.
These and still other objects and advantageous features of the present invention will become apparent from the following description of the preferred embodiment when taken in conjunction with the attached drawings.